This invention relates to adhesion of copper films to transition metal based underlayers in the manufacture of semiconductor devices.
The semiconductor industry is committed to introducing copper interconnects as a replacement for conventional aluminum and aluminum alloy interconnects in future generations of semiconductor devices. With its greater current carrying capacity, the introduction of copper interconnects should reduce device geometry, power consumption and heat generation. However, copper is a fast diffuser in silicon and drifts in dielectrics, resulting in a deterioration of devices at low temperatures. To avoid unwanted migration of copper atoms, a barrier layer or underlayer of a transition metal-based material, such as a tantalum-based material and more particularly tantalum or tantalum nitride, is typically used as a diffusion barrier between a copper interconnect layer and an underlying dielectric layer, such as a layer of silicon oxide. One method of providing the diffusion barrier is physical vapor deposition (PVD) by sputtering. However, sputter deposition, among other problems, cannot adequately cover the sidewalls of near-surface features having a high aspect ratio because sputtering is essentially a line-of-sight deposition process.
Two chemical vapor deposition (CVD) processes, thermal CVD and plasma-enhanced CVD (PECVD), are candidates to replace PVD. CVD provides highly uniform layers that conform to topographical features having high aspect ratios. Thermal CVD is a high temperature process in which a flow of gaseous reactants over a heated semiconductor substrate chemically react to deposit a solid layer on the heated substrate. Plasma-enhanced CVD is a relatively low-temperature process which introduces a plasma to activate the gaseous reactants.
To deposit a transition metal-based barrier layer, both CVD processes react a vapor-phase reactant, for example a transition metal halide reagent such as a tantalum halide or titanium halide, with a reducing gas, for example a hydrogen-containing gas such as either hydrogen (H2) or ammonia (NH3). If, for example, the reducing gas is hydrogen and the vapor-phase reactant is a tantalum halide, tantalum (Ta) is deposited, while tantalum nitride (TaNx) is deposited if the reducing gas is a nitrogen-containing gas, such as ammonia or a mixture of nitrogen and hydrogen.
The chemical reduction of the transition metal halide vapor-phase reactant produces halogen atoms as a by-product. The layer of transition metal-based material deposited by either of the CVD methods using a gas mixture comprising a transition metal halide vapor-phase reactant will incorporate a low residual level of the by-product halogen atoms as an unwanted impurity. For example, a layer of tantalum deposited on a substrate by plasma-enhanced CVD using, for example, tantalum pentafluoride will usually contain an average of about 0.5 atomic percent of the residual halide, in this instance the residual halide being fluorine. Residual fluorine concentration may peak, however, near interfaces, in particular the barrier/dielectric interface and the barrier/copper interface.
An elevated concentration of halogen atoms present at the barrier/dielectric interface has been found to correlate with a significantly reduced adhesion of the transition metal-based layer to the underlying dielectric. Likewise, elevated halogen atom concentration at the barrier/copper interface has been found to correlate with reduced adhesion of copper to the transition metal-based layer. Halogen atoms significantly disrupt the atomic bonding at the interfaces between the transition metal-based layer and the dielectric or copper film so that the transition metal-based layer and the overlying copper layer arc more likely to delaminate. A nitrogen-containing plasma pretreatment of the dielectric surface prior to CVD of the barrier layer has been proposed in copending application Ser. No. 09/723,876 entitled METHOD FOR PRETREATING DIELECTRIC LAYERS TO ENHANCE THE ADHESION OF CVD METAL LAYERS THERETO filed on even date herewith. The pretreatment provides nitrogen at the interface, which improves adhesion of a subsequently applied barrier layer. While this pretreatment is effective in improving adhesion at the barrier/dielectric interface, further improvement in adhesion is desirable at the barrier/copper interface.
There is thus a need for a CVD method that will prevent residual halogen impurities from altering the adhesion of a copper film deposited on a transition metal-based barrier layer deposited by a CVD process on a dielectric-covered substrate.
The present invention provides a method for improving the adhesion of copper films to transition metal based barrier underlayers. To this end, and in accordance with the present invention, transition metal based barrier underlayers are deposited by chemical vapor deposition using transition metal halide precursors. In one example of the present invention, a modulated Ta/TaNx underlayer is deposited using a tantalum pentafluoride precursor. The chemical reaction using these precursors generates halogen impurity atoms in the barrier layer. The barrier layer is consequently treated with a plasma generated from a nitrogen-containing gas, such as ammonia. The plasma post-treatment of the barrier layer reduces halogen impurity levels at the surface. Following the post-treatment, the copper film is deposited onto the treated barrier layer by physical vapor deposition. Due to the decreased halogen impurity level at the copper film/barrier layer interface, the copper film exhibits good adhesion to the barrier underlayer.